CO2 Cooling for an ATLAS upgrade
(or the CO2 conspiracy)
• Plant requirements
• CO2: Properties
• CO2: Consequences of the properties
• CO2: From LHCb to ATLAS
A.P.Colijn
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Requirements for ATLAS
1. Cool many distributed heat sources spread over
large volumes
2. Low material in the detector
3. Small temperature gradients over long distances
4. Radiation hard
5. Reliability
Will try to convince you that CO2 has it all!
A.P.Colijn
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Pressure [bar]
CO2 properties: p-H diagram
liquid
ΔH(-25C) = 280 kJ/kg
2-phase
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Enthalpy [kJ/kg]
P = 17 bar
gas
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C3F8 properties: p-H diagram
liquid
ΔH(-25C)=100 kJ/kg
P = 1.7 bar
2-phase
gas
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Run conditions @ -25C
C3F8
CO2
Pevaporation
1.7 bar
17 bar
ΔT for ΔP=+-0.1bar
+1.4 C / -1.5C
+0.2 C / -0.2 C
ΔT for ΔP=+-1.0bar
+12 C / ~-20 C
+1.8 C / -1.9 C
ΔH for evaporation
100 J/g
280 J/g
Flow for 100 W
1.0 g/sec
0.4 g/sec
Volume flow
0.6 cm3/sec
0.4 cm3/sec
Major difference for CO2 with respect to C3F8
cooling is the increase by a factor of 10 of the
evaporation pressure for T=-25C.
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High Pressure
CO2 plant must be able to withstand about
100bar (C3F8 in ATLAS 20bar)
+ low temperature gradients (dP/P small because P is high)
+ low tube diameter (because we can allow large dP)
+ low tube thickness (because of low tube diameter)
+ high tube flexibility (reduce mechanical stress)
- need more pipe at your heat sinks
- higher pressure = higher tube thickness
Some b.o.e. calculations to follow to get some feeling for
the numbers….
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Back-of-the-envelope-calculation
• What design consequences would it have to replace C3F8 with
CO2?
• Calculations assume CuNi pipes, just as for SCT
• Cool approximately 100W / cooling loop
• Disclaimer:
– calculations need more refinement
– calculations must be supported by measurements
A.P.Colijn
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Pipe diameter: pressure drop
Assume a pipe diameter of 0.9mm, which is 1/4th of the C3F8
cooling pipes we have now on the SCT endcap
8Fm L
P 
4
R
ΔP = pressure drop
Fm = mass flow
η
= viscosity
R
= pipe radius
L
= pipe lenght
Pressure drop for a 1g/sec CO2 flow is of the order of
0.08 bar. Even with a factor 10 more flow temperature
differences of a couple of degrees
A.P.Colijn can be expected!
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Pipe diameter: pipe thickness
2T S
d
Pmax
With the same pressure as in the
C3F8 system we could reduce
the pipe thickness by a factor
four
d
=
pipe diameter
T
=
pipe wall thickness
S
=
tensile strength
Pmax
=
max pressure
A CO2 system has to withstand
much higher pressure, so we
have to multiply the pipe
thickness again by a factor of six
T = 0.12 mm
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Material: CuNi pipes for CO2 and C3F8
m
L
4

2
2
d


(
T
 2d T )
C3 F8 / CO2
CuNi

II: CuNi for C3F8
at 20 bar
mI  0.015 g/cm

mII  0.18 g/cm
d=3.6mm
d=0.9mm
I: CuNi for CO2
at 100 bar
T = 0.07mm
T = 0.12mm
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Pipe flexibility: stress
Low diameter piping is much more
flexible: goes like R5. So no “funny”
bends needed in your structure to
absorb mechanical stress.
SCT EndcapA
disk1
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Heat absorption
P
L
 H d T
P = required cooling power
H = heat transfer coefficient in Wm-2K-1
(calculate/measure it… O(5000Wm-2K-1)
d = pipe diameter
ΔT=change in temperature
To absorb 10W you would need 4 cm of large diameter CuNi
pipe, versus 14 cm small diameter pipe if CO2 is used…….
Solutions R&D:
- “snaky” cooling pipes at cooling contact
- large contact area with module
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- or…. A.P.Colijn
CO2: From LHCb to ATLAS
1. Carbon copy the LHCb plant
–
–
–
(by then) tested technology
“Cold” input lines
Relatively small return lines
2. Design single-stage freezer
–
–
–
“Warm” input lines
Need oil-less CO2 compressor (!)
Need relatively big return lines
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CO2 plant: compressor vs pump based
Pressure [bar]
liquid
B
C C
B
A DD
E
2-phase
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Enthalpy [kJ/kg]
A
gas
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NIKHEF CO2 effort
• Interest from NIKHEF ATLAS group
– physicist (Fred Hartjes, Auke-Pieter Colijn, Els Koffeman)
– physicist/engineer (Bart Verlaat expertise…)
• LHCb cooling work finishes in near future
• We plan to setup CO2 test-bed at NIKHEF:
– for NIKHEF detector R&D (Gossip)
– for ATLAS upgrade development: support back-ofenvelope calculations of this talk with measurements
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